When the sky is light-pollution free and very dark it can be seen suffused by a faint glow, the airglow. Hold a hand against the sky and, if you are lucky, it will be visible as an inky black silhouette.

In the daytime and 80 km and higher in the thermosphere the sun's extreme ultra-violet radiation produces excited atoms and molecules . Airglow is the light emitted as the excitation is released. Here a red filtered all-sky camera has captured the airglow backed by the Milky-Way. The red airglow is from a narrow layer of excited hydroxyl (OH) radicals ~86 km high and possibly also from atomic oxygen emissions.

The glow is banded. In the video of shots at 20 minute intervals we see the progression of the bands against the wheeling stars. Gravity waves propagating upwards from the lower atmosphere generated them. They are parallel but appear by perspective to converge towards two diametrically opposite points.